Traditionally, a flexible cover can be used to cover a cargo bed of a pick-up truck for protecting the cargos stored in the cargo bed. The flexible cover can be made of fabric, flexible plastic, or other sheet material, such as canvas, vinyl plastic or other weather-resistant fabric. The flexible cover is commonly secured to a metal frame on the edge of the cargo bed by means of reversible attachment. Because the pick-up truck generally are used and stored outdoors, the fabric cover and the reversible attachment means to the metal frame are exposed to the moisture, extreme heat and cold, dust, mud, rocks and other external material transported in the cargo bed. The exposure to the external material causes problems for the covering and the structure used to reversibly attach the flexible cover to the metal frame.
For solving the foregoing problem, please refer now to FIGS. 1, 2 and 3, U.S. Pat. No. 6,575,520 discloses a traditional flexible tonneau cover system of a pick-up truck, wherein the flexible tonneau cover system is installed on a cargo bed 11 of a pick-up truck 10, and the flexible tonneau cover system comprises a pair of side rails 20, a flexible cover assembly 30, a rear header guide 40, a plurality of clamps 50, two hook-and-loop fastening assemblies 60 and a pair of tension control assemblies 70. The two side rails 20 are mounted on the top of two side walls 12 of the cargo bed 11 by the clamps 50; the flexible cover assembly 30 is mounted above a front wall 13 of the cargo bed 11; and the rear header guide 40 is mounted on the rear end of the two side rails 20 close to a tailgate 14 of the pick-up truck 10. The flexible cover assembly 30 has a front header 31, a flexible cover material 32 and a rear header (not-shown), wherein each end of the front header 31 has a screw member 311, wherein the screw member 311 has a head portion slidably installed in a groove 312 of the front header 31 and a thread rod portion installed in an oval-shaped hole 22 of the front end of the two side rails 20; the flexible cover material 32 can be rolled up on the front header 31 or be unrolled rearward on the two side rails 20 to cover the cargo bed 11; and the rear header can be engaged with or separated from the rear header guide 40. Each of the two hook-and-loop fastening assemblies 60 has a first fastener 61 and a second fastener 62. The first fastener 61 is attached to an elongated slat 63 by adhesives, chemical bonding, thermal fusion, or the like, wherein the elongated slat 63 is slidably inserted in a channel portion 21 of the side rail 20. Meanwhile, the second fastener 62 is attached to the flexible cover material 32 by stitching, adhesives, chemical bonding, fusion, and the like. The tension control assemblies 70 are fixed on the front end of the two side rails 20.
When the flexible cover material 32 is unrolled rearward on the two side rails 20 to cover the cargo bed 11, the first fastener 61 and the second fastener 62 can be engaged with each other to provide a weatherproof seal without gaps that will protect the cargos in the cargo bed 11 from rain, wind, and blowing debris. At the same time, the tension control assembly 70 is used to automatically control the tension of the flexible cover material 32 by elastic bias of a spring plunger therein or manually adjust the tension thereof by a user, wherein the front header 31 will shift either slightly forward or slightly back, so as to achieve the desired tension. The tension can be adjusted to accommodate expansion or contraction of the flexible cover material 32 in response to climatic conditions. When the flexible cover material 32 is slightly shifted forward or backward, the combination of the second fastener 62, the first fastener 61 and the elongated slat 63 is slid along the channel portion 21 of the side rail 20 for carrying out the corresponding position adjustment.
However, there are still problems existing in the actual use of the traditional flexible tonneau cover system, as follows: although the spring plunger of the tension control assembly 70 automatically pushes the front header 31 to control the tension of the flexible cover material 32, the extension degree of the spring plunger is limited. After long-term use, the length of the flexible cover material 32 may be permanently lengthened due to the elastic fatigue caused by the elastic bias of the spring plunger. Thus, the length of the flexible cover material 32 may be greater than the maximum extension degree of the spring plunger, so that the spring plunger can not stably abut against and hold the front header 31 and there will be a vibration noise problem generated between the front header 31 and the side rail 20 due to unstable installation. If the weather is too hot and the flexible cover material 32 is expanded too much, there will also be the vibration noise problem. On the other hand, the cost of the tension control assembly 70 is relatively high, and thus it is not advantageous to lower the price of the whole flexible tonneau cover system.
As a result, it is necessary to provide a tension control assembly for a flexible tonneau cover system of a pick-up truck to solve the problems existing in the conventional technologies, as described above.